I have Deco x60. 3 of them I installed in late 2021. 

My house has some dead zones and I have tried to move them to no avail.  I want to buy another unit or maybe 2 that matches these to expand the mesh. 

What do I buy please? Surely I don't just buy the same kit again as i don't need the 'main' unit just the annexes?  

Thanks in advance for all your help.  I am going to need very simple explanation and links please as I was at the limit of my understanding when I installed this back then!